ECs, also referred as supercapacitors, represent an attractive technology for energy storage and mobile power supply. ECs typically exhibit superior power density and cycle life, but with a relatively poor energy density at least one order of magnitude lower than those of traditional batteries. An electrode material is a central component of an EC, and can largely dictate its ultimate performance. Considerable efforts have been focused on developing electrode materials that can increase the energy density without sacrificing the power density or cycle life. Current ECs are typically constructed using porous activated carbon electrodes, typically with a gravimetric capacitance of about 80-120 F g−1 and a stack energy density of about 4-5 Wh kg−1, much lower than that of lead acid batteries (about 25-35 Wh kg−1).
Graphene has recently been investigated as an EC electrode material because of its high intrinsic electrical conductivity, excellent mechanical flexibility, an exceptionally large theoretical surface area of about 2630 m2 g−1, and a theoretical gravimetric capacitance of about 550 F g−1. However, because there is a strong π-π interaction between graphene sheets, they tend to re-stack to form graphite-like powders or films, which can severely decrease the surface area and reduce the ion diffusion rate, resulting in unsatisfactory gravimetric capacitances (typically <180 F g−1 in organic electrolytes) and relatively low charging/discharging rates.
Another figure-of-merit to evaluate an electrode material for ECs, in addition to gravimetric capacitance, is volumetric capacitance. There is typically a trade-off between gravimetric and volumetric capacitances for most electrode designs. For example, a highly porous electrode can offer a large specific surface area and can favor ion diffusion for high gravimetric capacitance, but may have a lower volumetric capacitance due to its relatively low packing density. On the other hand, a more compact electrode can boost the volumetric capacitance but decrease the ion-accessible surface area and ion diffusion rate, resulting in a lower gravimetric capacitance and poor rate performance. Therefore, there is a formidable challenge to both achieve high gravimetric and volumetric capacitances while retaining excellent rate capability, which is desired for the development of practical ECs with high energy and power densities.
It is against this background that a need arose to develop described herein.